1. Field of the Invention
The present invention relates to a connector unit for differential transmission.
2. Description of the Related Art
There are two types of data transmission methods: a normal transmission method and a differential transmission method. The normal transmission method employs an electric wire for each data item. The differential transmission method, using a pair of electric wires for each data item, simultaneously transmits a “+” signal to be transmitted and a “−” signal equal in magnitude and opposite in direction to the “+” signal. The differential transmission method, which has the advantage of being less susceptible to noise compared with the normal transmission method, has been used more widely.
A connector is used to transmit data between apparatuses. In order to form a data path for differential transmission between the apparatuses, a connector for differential transmission (a differential transmission connector) having a special structure is used. Compared with normal connectors, the differential transmission connector has a complicated structure. However, the differential transmission connector is required to have the same insertion and extraction durability as that of normal connectors. Here, the term “insertion and extraction durability” refers to the number of times a cable connector is inserted into (and connected to) and extracted from a socket connector which number can still guarantee stable differential transmission in the case of repeated insertion and extraction operations.
FIGS. 1 and 2 are schematic diagrams illustrating a conventional differential transmission connector unit 10. The differential transmission connector unit 10 includes a cable connector 20 at a cable end and a socket connector 30 to be mounted on a printed board. In FIGS. 1 and 2, X1-X2 represents the X-axial directions (the directions of the row of contact alignment or the directions of connector width), Z1-Z2 represents the Z-axial directions (the directions of the column of contact alignment or the directions of connector height, and Y1-Y2 represents the Y-axial directions (the directions of contact length, the directions of connector depth, or the directions of connector insertion and extraction). This representation of directions is equally applied to all drawings illustrating embodiments of the present invention. FIG. 1 illustrates a state where the contacts of the cable connector 20 and the contacts of the socket connector 30 oppose each other. FIG. 2 illustrates a state where the cable connector 20 is inserted in and connected to the socket connector 30 so that the contacts of the cable connector 20 are connected to the corresponding contacts of the socket connector 30.
In the socket connector 30, signal contact pairs, each formed of a first signal contact 31 and a second signal contact 32 arranged in the Z-axial directions, and ground contacts 33 are incorporated in an electrically insulating block body 40 illustrated in FIGS. 3A and 3B so as to be arranged alternately with each other in the X-axial directions with a pitch p, being entirely surrounded by a shield cover (not graphically illustrated).
Each of the first and second signal contacts 31 and 32 has a long and narrow shape. Each ground contact 33 has a plate-like shape, and includes a main body part 33a and a rectangular projection part 33b projecting in the Y2 direction from the main body part 33a. The projection part 33b includes a cutout part 33c formed at the end of the projection part 33b. 
The socket connector 30 is mounted on a printed board so that each pair of the first and second signal contacts 31 and 32 is connected to a corresponding pair of wiring patterns and the ground contacts 33 are connected to corresponding ground patterns so as to be set to ground potential. Each ground contact 33 has a plate-like shape and provides a shield between the signal contact pair (the first and second signal contacts 31 and 32) on one side of the ground contact 33 and the signal contact pair on the other side of the ground contact 33.
In the cable connector 20, signal contact pairs, each formed of a first signal contact 21 and a second signal contact 22 arranged in the Z-axial directions, and ground contacts 23 are incorporated in an electrically insulating block body (not graphically illustrated) so as to be arranged alternately with each other in the X-axial directions, being entirely surrounded by a shield cover (not graphically illustrated). Each first signal contact 21 includes a plate part 21a and a finger part 21b extending in the Y1 direction from the plate part 21a. Each second signal contact 22 includes a plate part 22a and a finger part 22b extending in the Y1 direction from the plate part 22a. Each ground contact 23 includes a plate part 23a and a fork part 23b formed of a pair of finger parts extending in the Y1 direction from the plate part 23a. 
The cable connector 20 is connected to an end of a differential transmission cable containing multiple pairs of wires. Each pair of wires includes a first signal wire, a second signal wire, and a drain wire. The first and second signal contacts 21 and 22 of each signal contact pair are connected to the first signal wire and the second signal wire of the corresponding pair of wires. Each ground contact 23 is connected to the drain wire of the corresponding pair of wires. Each ground contact 23 has a plate-like shape and provides a shield between the signal contact pair (the first and second signal contacts 21 and 22) on one side of the ground contact 23 and the signal contact pair on the other side of the ground contact 23.
The cable connector 20 is inserted into the socket connector 30 in the Y1 direction so as to be connected thereto as illustrated in FIG. 2. A contact surface 21c of the finger part 21b of each first signal contact 21 of the cable connector 20 rubs on an upper surface 31a of the corresponding first signal contact 31 of the socket connector 30 so as to come into contact therewith. A contact surface 22c of the finger part 22b of each second signal contact 22 of the cable connector 20 rubs on a lower surface 32a of the corresponding second signal contact 32 of the socket connector 30 so as to come into contact therewith. Contact surfaces 23c and 23d of the fork part 23b of each ground contact 23 of the cable connector 20 rub on an upper end surface 33d and a lower end surface 33e, respectively, of the projection part 33b of the corresponding ground contact 33 of the socket connector 30 so as to come into contact therewith.
Each first signal contact 21 and the corresponding first signal contact 31 have a “+” signal transmitted thereto. Each second signal contact 22 and the corresponding second signal contact 32 have a “−” signal transmitted thereto. Each first signal contact 21 and the corresponding signal contact 31 and each second signal contact 22 and the corresponding signal contact 32 are shielded by the corresponding ground contacts 23 and 33 from the adjacent first signal contact 21 and the corresponding signal contact 31 and the adjacent second signal contact 22 and the corresponding signal contact 32 along the X-axis. Further, the signals equal in magnitude and opposite in direction are transmitted to each first signal contact 21 and the corresponding signal contact 31 and each second signal contact 22 and the corresponding signal contact 32. Accordingly, a virtual ground plane is formed between the first signal contacts 21 and 31 and the second signal contacts 22 and 32. As a result, the “+” and “−” signals are transmitted in a state less susceptible to noise in any part of the connected cable connector 20 and socket connector 30.
When the cable connector 20 is pulled in the Y2 direction, each finger part 21b rubs on the corresponding first signal contact 31, each finger part 22b rubs on the corresponding second signal contact 32, and each fork part 23b rubs on the corresponding projection part 33b so that the cable connector 20 is extracted from the socket connector 30. Japanese Laid-Open Patent Application No. 2000-068006 discloses a conventional differential transmission connector.
The inventors of the present invention evaluated the insertion and extraction durability of the differential transmission connector unit 10. The evaluation was performed by repeating insertion and extraction to measure the differential transmission characteristic of a signal, and recording how the differential transmission characteristic of the signal decreased. As a result, it was found that the differential transmission characteristic of the signal decreased when the number of repetitions of insertion and extraction exceeded a predetermined value.
As a result of observing damage caused to the contact portion of the differential transmission connector unit 10 whose differential transmission characteristic decreased due to the repeated insertion and extraction, the contact portion of the ground contacts 23 and 33 was found to be more damaged than the contact portion of the first and second signal contacts 21 and 22 and the first and second signal contacts 31 and 32.
The reason is considered in the following.
First, a description is given of the process of manufacturing the first signal contacts 31, the second signal contacts 32, and the ground contacts 33 of the socket connector 30.
As illustrated in FIG. 4, a semi-finished product 52 in which the first signal contacts 31 are arranged like comb teeth on a belt part 51 is stamped out by press working from a copper-alloy plate material 50 rolled by a roller. Then, the first signal contacts 31 are bent by press working, subjected to gold-plating, and cut off from the belt part 51 as finished products. The upper surface 31a of each first signal contact 31 is a rolled surface subjected to the rolling by the roller.
As illustrated in FIG. 5, a semi-finished product 62 in which the second signal contacts 32 are arranged like comb teeth on a belt part 61 is stamped out by press working from a copper-alloy plate material 60 rolled by a roller. Then, the second signal contacts 32 are bent by press working, subjected to gold-plating, and cut off from the belt part 61 as finished products. The lower surface 32a of each second signal contact 32 is a rolled surface subjected to the rolling by the roller.
As illustrated in FIG. 6, a semi-finished product 72 in which the ground contacts 33 are arranged like comb teeth on a belt part 71 is stamped out by press working from a copper-alloy plate material 70 rolled by a roller. Then, the ground contacts 33 are subjected to gold-plating and cut off from the belt part 71 as finished products. The upper end surface 33d and the lower end surface 33e of the projecting part 33b of each ground contact 33 are fracture surfaces due to the press working.
Next, a description is given of the process of manufacturing the first signal contacts 21, the second signal contacts 22, and the ground contacts 23 of the cable connector 20.
As illustrated in FIG. 7, a semi-finished product 82 in which the first and second signal contacts 21 and 22 are arranged like comb teeth on a belt part 81 is stamped out by press working from a copper-alloy plate material 80 rolled by a roller. Then, the first and second signal contacts 21 and 22 are subjected to gold-plating and cut off from the belt part 81 as finished products. The contact surface 21c of the finger part 21b of each first signal contact 21 and the contact surface 22c of the finger part 22b of each second signal contact 22 are fracture surfaces due to the press working.
As illustrated in FIG. 8, a semi-finished product 92 in which the ground contacts 23 are arranged like comb teeth on a belt part 91 is stamped out by press working from a copper-alloy plate material 90 rolled by a roller. Then, the ground contacts 23 are subjected to gold-plating and cut off from the belt part 91 as finished products. The opposing contact surfaces 23c and 23d of the fork part 23b of each ground contact 23 are fracture surfaces due to the press working.
Here, the fracture surfaces due to press working were found to be considerably rough compared with rolled surfaces, and it was found that the gold plating layer on the fracture surfaces rubs off easily compared with that on rolled surfaces.
Referring again to FIGS. 1 and 2, the fracture contact surfaces 21c and 22c of the first and second signal contacts 21 and 22 rub on the rolled upper and lower surfaces 31a and 32a of the first and second signal contacts 31 and 32, respectively. On the other hand, the fracture contact surfaces 23c and 23d of the ground contacts 23 rub on the fracture upper and lower end surfaces 33d and 33e, respectively, of the ground contacts 33.
Since the fracture surfaces rub on each other, the gold plating layer of each of the ground contacts 23 and 33 is scraped off considerably so that the base surface is exposed so as to increase the contact resistance of the contact part, which was found out to be the reason why the insertion and extraction durability is prevented from increasing.